In order to provide a television with a screen size greater than approximately 40 inches a display device other than a direct view cathode ray tube (CRT) is typically used. As the screen size of a CRT increases, so too does the depth. It is generally accepted that for screen sizes greater than 40 inches direct view CRTs are no longer practical. Two alternatives exist for large screen (>40 inch screen size) displays: projection displays and plasma displays.
Current plasma displays are much more expensive than projection displays. Plasma displays are generally thin enough to mount on a wall, but can be heavy enough that mounting can be difficult. For example, current 42 inch plasma displays can weigh 80 pounds or more and 60 inch plasma displays can weigh 150 pounds or more. One advantage of plasma displays over current projection displays is that plasma displays are typically much thinner than current projection displays having the same screen size.
Projection displays, specifically rear projection displays, are typically more cost-effective then plasma displays. Projection displays may also consume too much space in a room to provide a practical solution for large screen needs. For example, typical 60 inch rear projection displays are 24 inches thick and can weigh 200 to 300 pounds.
FIG. 1 illustrates a prior art rear projection display device. In general, display device 100 includes optical engine 140, projection lens 130, back plate mirror 120 and screen 110. Optical engine 140 generates an image to be projected on screen 110. Projection lens 130 projects the image from optical engine 140 on to back plate mirror 120, which reflects the image to screen 110. The size of display device 100 is proportional to the size of the image to be displayed on screen 110. Thus, for large screen sizes (e.g., >60 inches), the overall size of display device 100 can be very large.
Thin rear projection display devices have been developed that are less than 12 inches thick. However, these thinner rear projection display devices typically rely on an aspherical mirror, which is difficult to manufacture and difficult to align. The difficulties associated with the aspherical mirror results in current thin rear projection displays being expensive, which restricts the availability of rear projection displays in desirable packages.
FIG. 2 illustrates a prior art thin rear projection display device with an aspherical mirror. An image from optical engine 260 is projected on reflective mirror 240 by projection lens 250. Reflective mirror 240 reflects the image to aspherical mirror 230, which magnifies the projected image and extends the field ray angle. Aspherical mirror 230 reflects the image to back plate mirror 220, which then reflects the image to screen 210. While rear projection display device 200 provides a thinner package for the same size screen as compared to display device 100 of FIG. 1, the manufacturing and alignment issues associated with use of aspherical mirror 230 greatly increases the cost of display device 200.
Another shortcoming of display device 200 is the angle of optical engine 260 with respect to mirrors 220, 230 and 240 and to screen 210. Without compensation, the angle of optical engine 260 results in a trapezoidal, or keystone, shaped image. The compensation associated with the angle to provide a square image further increases the cost and complexity of display device 200.